Denitrifying bacteria are important agents in managing waste. One of the electron transfer pathways in which nitrates are converted eventually to N2 utilizes, for the particularly potent denitrifying bacteria Alcaligenes faecalis Strain S-6, a copper containing nitrite reductase, and a blue protein which while normally acting as the agent which transfers an electron to the nitrite reductase, can also act as an inhibitor in the presence of O2. I propose to determine the structure of that blue copper protein to high resolution in both the oxidized and reduced forms, using standard techniques of X-ray diffraction. The long term objectives are to understand electron transfer at a molecular level, to understand what it is about electron transfer protein structures that facilitates transfer, what provides specificity, how different proteins make use of the same cofactor. Partial sequence information and the amino acid composition of this blue protein of molecular weight intermediate to two other blue copper proteins with known structure, plastocyanin and azurin, indicates that while A. faecalis blue protein probably has the same chromophore, it may have different folding, and may well represent a structural subclass of these Type I blue copper proteins. Thus this structure determination will afford a new view of how an organism solubilizes, utilizes and regulates an electron transfer cofactor. Moreover, since its interaction with the nitrite reductase is well defined, and its structure is also being worked on, we can eventually directly examine the question of how electron transfer components interact.